They were far from conventional, Dean knew. This weird relationship that had started years ago as hatred, manipulation, and Heavenly orders. How it had morphed into this… whatever it is… Dean had no idea. They had died and killed and changed irreversibly for each other and neither had ever thought twice about it, not once, not since the day Castiel walked in all Angelic grace and general kickassness, saving his life effortlessly.
Castiel, Angel of the Lord, quickly became just Cas, the Winchester's guardian angel, Dean's best friend. That was an interesting transition. Dean, as a general rule, didn't have many friends. In his line of work, it was dangerous to have them. They tended to end up dead. But Cas, Cas was hard, almost impossible, to kill. He was a goddamn angel. Sure, he was mostly clueless about things like personal space and pop culture, but he wasn't going anywhere. Dean had the time to teach him everything that he deemed important to know.
Cas, the guardian angel then became, Cas, honorary Winchester, and that, too, was an interesting transition. He was no longer in the "we trust you not to kill us" place that Crowley tended to occupy against everyone's better judgment and was firmly in the "we will do anything to save you" place that was saved only for the brothers. Dean wasn't sure when that happened, or how, or why, but he knew that it had. Sammy would always come first, but Cas, Cas was a very close second. Honestly, he was just hoping that the day that he would have to choose between the two would never come.
He always acted like the comments about him and Cas being together bothered him. That wasn't really a lie. They did bother him, in that way that digs down into your soul and tells you that it should be true, They bothered him because they seemed just as natural as Sammy being a nerd, or Bobby an alcoholic. They bothered him because despite that, they weren't true. And the fact that maybe, deep, deep down, he wanted them to be. But God knows that Dean Winchester wasn't going to admit that to anyone (God did know, and it kind of almost made him regret giving the man free will).
Castiel didn't understand at first what he was feeling towards the crass man that he was told to raise from Hell. Impatience was certainly the first thing that he could name. Then came the disbelief, which was so very new for him. How could Dean Winchester, the Righteous Man, not think that he deserved to be saved? It astounded him as it made no sense. The only reason that Dean ever stepped foot in Hell was to save Sam. Admittedly, saving Sam caused a million problems in and of itself, but good intentions.
That became a phrase that Castiel found himself clinging to as he began to grow with the Winchesters—good intentions. Sam was a fascinating specimen of his Father's creation. The boy—man, by humanity's standards—had nothing but good intentions. In that way, he and Castiel were the same. All logic stood to say that Sam would be the one that he would connect with most easily. Castiel liked Sam, of course, and they were friends. But there was Dean. Dean who would do anything for his little brother Sammy. Dean who wanted to save as many people as he could. Dean who took pride in his work, no matter how little recognition he got for it. Dean who was the closest thing to an angel a simple human could be. It was a funny sort situation that they found themselves in, the most human angel and the most divine human.
Eventually, Castiel figured out that it was affection, and love, that he felt for the oldest Winchester. Whether it was familial or romantic, he wasn't entirely sure. But that was ultimately not important. What was important to Castiel was that he was always by Dean's side, or at least always on that list of people that he trusted. That they were Sam and Dean and Cas. That would be enough until he figured out exactly what type of love this was.
Sam held the firm belief that Dean was an idiot and Cas was an oblivious child. The amount of staring and touching and time alone with each other made it painfully obvious to Sam what they both wanted, but Dean was no stranger to denial and Cas wasn't always the best with human traditions and social cues.
He was waiting patiently for the day when something concrete finally happened between them. Maybe then he wouldn't always be in the middle of all that tension.